This invention relates to crossmixing methods for ensuring adequate mixing of developer particles in an electrostatic printing machine.
The development of electrostatic images is well known in the art, and is most often accomplished by employing a two component developer consisting of a larger particle called a carrier, ranging upwardly from 75 micron, and a smaller particle, called toner, usually about 5 to 10 microns in diameter. These two particles are selected to that their surfaces interact triboelectrically to create the proper sign of charge on the toner particle necessary for development of the image. In some development systems, additives may be added to the toner consisting of agents for modifying the charge, assisting in the cleaning, and reducing Van der Wahl forces between the toner and carrier.
Two methods of carrier development are in common use today. These are magnetic brush development and cascade development. The primary difference between the two development systems is the motive force for bringing the developer into contact with the latent electrostatic image. In the case of a magnetic brush system, the force is magnetic and this requires that the carrier be magnetic. In the case of cascade, the carrier has no such requirement but in many cases it may be magnetic.
In the operation of these development systems, it is required that the composition of the developer be uniform throughout the developer system in order that the density of the copy is uniform across the surface. Since, in the operation of the machine, the image area is not uniformly distributed across the surface of the copy, consumption of the toner is not uniform across the width of the developer unit. In more complex developers (those consisting of more than 2 discrete components), the problem may be further complicated by the components not being consumed at similar rates; in addition to the toner/carrier ratio changing, the amount of additive or additives at various points across the developer roll may vary and affect the performance. Many methods have been proposed to correct this problem, two examples being presented in U.S. Pat. Nos. 3,707,947 and 3,724,422. The method used in both of these developer systems is a series of oppositely sloped channels through which the developer flows. These channels are arranged to move one half of the developer flow in one direction, side to side, while the other half moves in the opposite direction. These systems rely entirely on gravity to accomplish this side to side motion. In addition to this side to side motion, the flow in these diverters serves to agitate the developer and stabilize the triboelectric charging.
A simpler method of mixing and stirring the developer is shown in U.S. Pat. No. 3,575,139, and consists of a pair of augers positioned in the developer sump and driven so as to move them in opposite directions. This latter method causes high shear forces to develop in the sump area and can materially shorten the life of the developer.
The present invention is a method wherein mechanical movement and mixing of developer particles is achieved through use of a single screw conveyor having both left-hand and right-hand threads. In this fashion particles of developer material move in opposite directions along the screw conveyor. Advantageously, developer material is introduced at opposite ends into the screw conveyor and is moved therealong to the mid-section thereof, from whence it is discharged in a direction toward the opposite end of the screw conveyor. It discharges into the sump of the printing machine, from which the particles are removed by conventional magnetic brushes and deposited upon a plate assembly. The plate assembly includes separators for guiding the developer particles, and the separators are angled in one direction on one lengthwise portion of the plate assembly and in another direction on another lengthwise portion, so as to shift particles of developer material in opposite lengthwise senses on the two portions of the plate assembly. The plate assembly is positioned over the screw conveyor, and includes openings adjacent opposite ends of the conveyor so that particles of developer material on the plate assembly drop directly into the screw conveyor and are conveyed along the conveyor as just described. The remainder of the particles on the plate assembly, being shifted as just described, cascade downwardly from that assembly in a curtain into the sump of the printing machine from which they are removed by the magnetic brushes.
By this action, a developer particle moving continuously through the system may be considered as moving in a helical path commencing from a mid-section of the printing machine to one end thereof, at which point it enters the screw conveyor and moves into the central portion of the sump, being directed toward the opposite end of the machine. At this central portion of the sump the particle is again moved in a helical path toward the opposite end of the machine, being deposited this time at the opposite end of the screw conveyor in which it moves to the central part of the sump to continue again in the helical path it initially took.
Tests were performed with a system embodying the present invention with an unbalanced developer, loading 3% concentration developer in one side and 1% concentration in the other and sampling the developer after 1 minute, 2 minutes and 3 minutes of running time. After 3 minutes of running time, the toner concentration was uniform across the unit. In imaging tests in an unbalanced operation (imaging on only 8% of the width of the unit), the toner concentration in the unused section of the developer has varied no more than 2% to 3% of the total toner concentration at a level of 2%. The density reproduction of the copy across the unit was no more than 0.05 units at a 2.0 optical density level. In this test, the developer contained a toner with three discrete components, and all components of the toner and carrier were maintained in the proper ratios throughout the developer unit.
The mixing and transporting action of the screw conveyor handles only about 10% of the developer at one time. This small percentage in the screw conveyor reduces the "wear" generated by the system. In experiments performed, the developer life has not been found to be reduced by such a crossmixer system, and in one series of experiments an experimental developer has produced 80,000 copies with not evidence of failure with a charge of only 4 pounds of developer. This compares well with some prior art systems in which 20 pounds of developer provides approximately 200,000 copies.
The invention will be more completely understood by reference to the following detailed description, taken in conjunction with the appended drawings.